Semiconductor laser oscillators (i.e., laser light sources) and semiconductor laser amplifiers were first developed in the 1960s. Such laser oscillators and amplifiers offered the obvious advantage of extremely small size over other types of lasers. (A typical semiconductor amplifier may be on the order of few hundred micrometers long). These first semiconductor lasers were fabricated of a single type of semiconductor.
A modern semiconductor laser oscillator or amplifier typically comprises a semiconductor heterostructure, that is, it is made from more than one semiconductor material such as gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). Semiconductor oscillators and amplifiers are made from a combination of semiconductor materials which have different bandgap energies to achieve amplifiers are made from a combination of semiconductor materials which have different bandgap energies to achieve electrical carrier confinement as well as different optical indices of refraction to achieve optical confinement.
Many approaches have been proposed to achieve the goal of high-power, continuous-wave operation with a single-lobed spatial mode output from semiconductor lasers or semiconductor amplifiers. One such approach is to employ a laterally tapered gain region.
An exemplary double heterostructure tapered amplifier of the prior art is comprised of three layers of semiconductor material; 1) a p-type material with a relatively high bandgap, such as AlGaAs, 2) and n-type material with relatively high bandgap, which may also be AlGaAs, and 3) a relatively low bandgap p-type material such as GaAs, sandwiched between the other two layers and comprising the active region of the amplifier.
In general, any direct-band-gap semiconductor ternary or quaternary alloy system (such as AlGaAs or InGaAsP whose various alloys can be selected to have lattice constants close to that of the growth substrate crystal can be used for laser amplifiers or oscillators.
A linearly tapered gain region is formed in the active layer. Commonly, the gain region is formed by etching through the active layer and regrowing another semiconductor layer in the etched region. A metal contact pad is placed in electrical contact with the top surface of the top layer commensurate with the underlying gain region. When sufficient current is passed through the metal contact, electrons and holes are injected into the active layer in the gain region from the high bandgap material layers. These electrons and holes are trapped in the potential well created by the low bandgap GaAs material. Since the electrons are trapped in the active region they are forced to combine with each other in the GaAs material. Light introduced into this region is amplified.
Confinement of the light around the GaAs active layer is provided by the wave guide properties of the AlGaAs/GaAs/AlGaAs material structure. The AlGaAs layers have a lower optical index of refraction than that of the GaAs material thus providing total internal reflection of light off the interfaces so that most of the light remains within the GaAs layer allowing the active layer to act essentially as a wave guiding layer.
In operation, substantially diffraction limited light generated, for example, by a low power laser is focused by a lens system onto the input facet of the tapered amplifier. If the beam is allowed to spread naturally without interference, the beam will remain diffraction-limited as it spreads, thus leading to the desired single-mode amplified output beam. The expansion of the beam reduces the possibility of optical damage at the output facet because the power in the beam is more spread out.
Due to the high gain achieved by semiconductor optical amplifiers, such amplifiers are easily susceptible to self-oscillation. Self-oscillation occurs when a small portion of the light striking the output facet is reflected back into the semiconductor medium. This reflected light is further amplified and a portion reflected again off of the input facet. If the total round trip product of amplification gain and reflection loss reaches unity, self-oscillation occurs. In this case, self-oscillation will build up from internal spontaneous emission, even in the absence of externally injected light.
In the case of semiconductor laser oscillators as opposed to amplifiers, self-oscillation is necessary and, in fact, constitutes laser action. However, self-oscillation is undesirable in semiconductor amplifiers because it interferes with the amplification of the input light and may degrade the output mode quality as well as reduce gain.
This problem can be partially alleviated by using anti-reflection coatings on the input and output facets 20 and 22. However, a sufficiently small residual reflectivity is often difficult to achieve in practice and, in fact, may be impossible to incorporate in certain monolithic implementations where a MOPA is integrated on a chip. The problem is particularly severe in long amplifiers where the larger gains achieved can easily overcome very small reflection coefficients.
Examples of the state of the art of tapered semiconductor laser amplifiers are Bendelli, G., Komori, K., Arai, S., and Suematsu, Y., A New Structure For High-Power TW-SLA, IEEE Photonics Technology Letters, Vol. 3, No. 1, Jan. 1991, which discloses an exponentially tapered semiconductor laser amplifier having a high refractive index gradient at the boundaries of the tapered gain region; and Yazaki, P., Komori, K., Bendelli, G., Arai, S., and Suematsu, Y., A GaInAsP/InP Tapered Waveguide Semiconductor Laser Amplifier Integrated with a 1.5 .mu.m Distributed Feedback Laser, IEEE Transactions Photonics Technology Letters, Vol. 3, No. 12, Dec., 1991, which discloses an exponentially tapered wave guide semiconductor laser amplifier monolithically constructed with a distributed feedback laser. The Yazaki, et al. device also has a high refractive index gradient at the boundaries of the gain region.